Piezoelectric effects of single-crystal GaAs and multi-layered Al x. As/GaAs material measured by the Michelson interferometer

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1 ORIGINAL ARTICLE and multi-layered Al x As/GaAs material measured by the Michelson interferometer Panya Kheanumkeaw 1 Supasarote Muensit 2 and Patara Aiyarak 3 Abstract: Kheanumkeaw, P., Muensit, S. and Aiyarak, P. and multi-layered Al x As/GaAs material measured by the Michelson interferometer Songklanakarin J. Sci. Technol., 2003, 25(5) : The inverse piezoelectric effect, in which the strains were electrically induced, in a single crystal of GaAs and in a multilayer structure of Al x As/GaAs was measured using a simple optical system, i.e., Michelson interferometer. An ac driving voltage was applied to the sample to produce a change in the order of m in sample thickness. These changes were detected by the optical system to give the sample displacement as a function of applied driving voltage. The slope of the plot of this relationship led to the piezoelectric coefficients of (2.8±0.1) and (3.9±0.1) m/v for GaAs and Al x As/GaAs, respectively. The first agreed well with reported values and the latter was the first report for Al x As/GaAs. Owing to the equality for the inverse effect and the direct effect, in which an electric field can be mechanically induced, it is anticipated that in the absence of external electric field, the internal piezoelectric field can be induced in the multi-layered semiconductor. Key words : piezoelectric, interferometer, semiconductor 1 M.Sc.Student (in Physics), 2 Ph.D.(Materials Physics), Asst.Prof., 3 Ph.D.(Optoelectronics), Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla Thailand. Corresponding msupasar@ratree.psu.ac.th Received, 26 May 2003 Accepted, 8 July 2003

2 Vol. 25 No. 5 Sep.-Oct àõ ªí πè âÿ À Ëπ Ï Õ å ª Ø å æõ Õ Á µ Ë«â«πÕ π Õ å øõ å µõ å Õßº Ë «Õ å π å «ÿà Èπ ÕßÕ Ÿ π - Õ å π å Õß Õ å π å «. ß π π å « (5) : ª Ø å æõ Õ Á µ Ëß «Ÿ Àπ Ë «π Àâ Èπ â â«π øøñ πº Ë «Õß Õ å π å (GaAs) π ß â ßÀ Èπ ÕßÕ Ÿ π - Õ å π å Õß Õ å π å (Al x As/GaAs) â µ ««â«ß ßÕ à ßßà Ë «à πõ π Õ å øõ å - µõ å ËÕ» å øøñ Ÿ ªÑÕπ Àâ à µ «Õ à ß Àâ ª Ë π ª ß π µ π π««àπ Õßµ «Õ à ß ª Ë π ª ßπ Èµ «â â«ß ß â ßµâπ Àâ â Õßµ «Õ à ß ªìπøíß å π» å øøñ ËªÑÕπ «π Õß ø «æ π åπ Èπ ª Ÿà à ß Ë æõ Õ Á µ à ( ) ( ) m/v À GaAs Al x As/GaAs µ à Õ âõß à Ë ß π à À ß ªìπ ß π Èß À Al x As/GaAs «à π Õßª Ø å æõ Õ Á µ µ ß Ëß π øøñ Ÿ Àπ Ë «π â««ß Àâ â«à âª» π øøñ πõ π æõ Õ Á µ π Á Èπ â π Ëßµ «π À Èπ «øî å µ å À «ß π π å Õ ÕÀ À à ßÀ«ß A complete set of material parameters for semiconductors such as gallium arsenide (GaAs) and Al x As has been reviewed elsewhere (Blakemore, 1982; Adachi, 1985). Obviously, sets of theoretical values of the various piezoelectric coefficients for semiconducting materials have been reported whereas experimental values of the coefficients appear to be lacking. The present work made use of the inverse piezoelectric effect for measuring the coefficients, with the strain being measured by optical interferometry. By using this technique, the conductivity problems possibly caused by charge migration within a semiconductor could be avoided. GaAs crystallizes in the zinc-blende structure, which is the simplest crystal lacking a center of symmetry and, hence, capable of exhibiting piezoelectric behaviour (Adachi, 1992). The piezoelectric tensor in zinc-blend crystal has the form: d d = d 14 0 (1) d 14 The only non-zero component, d 14 is the piezoelectric strain coefficient which is connected with the piezoelectric coefficient obtained from a thickness expansion, d 33 as (Mason, 1964) d 33 = d 14 3 (2) By applying an ac electric field across a thickness, the d 33 value can be deduced from (Zhang et al., 1988) d 33 = d ac V (3)

3 Vol. 25 No. 5 Sep.-Oct where d ac is the resulting field-induced mechanical displacement in the direction normal to the crystal plane and V is the applied driving voltage. For Al x As, which is most commonly prepared in a form of thin films grown on zincblend structure substrates, the material parameters, e.g., dielectric constant, ionization energy and piezoelectric coefficient, were calculated as a function of x (Adachi, 1985). As this material is of importance for electron- and wave-device applications, accurate experimental measurements of the piezoelectric coefficient should have significant value. Materials and Methods The GaAs samples used in this work were cut into 4 mm 4 mm from a 0.3 mm-thick commercial (111)B-n + wafer. The Al x As/GaAs sample was prepared by the metalorganic chemical vapour deposition (MOCVD) (Shur, 1990). A sequence of the layers as shown in Table 1. As a check on the influence of the electrode material on the measured piezoelectric response, three different metals, namely aluminum (Al), gold (Au) and indium-gold (In-Au) were tested. Each of them was evaporated over an area of a diameter of 0.2 mm of the sample surface for using as electrical contacts. Electroded samples were checked for current-voltage (I-V) characteristics using the HP 4140B pa Meter/DC Voltage Source. Table 1. Layer sequences of the Al x As sample. Thickness Layer x Type (10-10 m) 500 GaAs n 2000 Al x As 0.1 n 1000 Al x As GaAs 1000 Al x As Al x As 0.3 p 3000 GaAs p In the Michelson interferometric arrangement (Muensit and Kheanumkeaw, 2002), the incoming laser from a Uniphase 1135P He-Ne laser source was divided into two beams by a beam splitter. One of the beams was reflected from a reference mirror mounted on a piezoelectric transducer and the other beam from the electrode on the free surface of the sample. The other surface of the sample was glued with conducting epoxy to a brass plate. An ac field was applied to the sample with electrical contacts arranged in a manner that the thickness excitation measurement (IEEE Standard on Piezoelectricity, 1988) could be performed. The reflected laser beams combined at the beam splitter and produced an interference pattern. A pin photodiode detector was used to detect the changes in light intensity of the interference pattern. The detector voltage was measured by a SR 530 Lock-in Amplifier (Stanford Research Systems) and converted into the displacement of the sample surface. At a constant driving frequency, the driving voltage was varied and measurement was repeated over the frequencies above 1 khz. Results and Discussion A typical I-V curve was observed for GaAs with indium-gold as shown in Figure 1. The linear characteristic of the curve resulted from an ohmic contact between the metal and the sample surface (Sze, 1969). For the Al x As/GaAs sample with indium-gold, a p-n junction characteristic was obtained. In the following measurements, indiumgold was chosen as the electrodes for all samples. The interferometric measurements were repeatable over the frequency range from 5 khz to 20 khz for both GaAs (Figure 2) and Al x As/ GaAs (Figure 3). For each sample, a linear response was observed over the driving voltages ranged from 1 V to 10 V at a constant driving frequency. A minimum value of the sample displacement was in the order of m. The slope of the plot for GaAs was (1.6±0.1) m/v. From equation (2), the slope gave the d 14 coefficient to be (2.8±0.1) m/v. This value was about 5%

4 Vol. 25 No. 5 Sep.-Oct Figure 1. A typical I-V curve for GaAs with indium-gold electrode. Figure 2. Avearage displacement over the frequencies ranged from 5 khz to 20 khz as a function of driving voltage for GaAs with indium-gold electrode. different from the values reported in the literature (Charlson and Mott, 1963; Arlt and Quadflieg, 1968). The slope of the plot for Al x As/GaAs gave the d 14 coefficient to be (3.9±0.1) m/v. A theoretical prediction of a value of the d 14 coefficient for Al x As/GaAs (Adachi, 1985) is d 14 (x) = x. By this equation, the measured d 14 coefficient for Al x As/GaAs in this work corresponded with x = It could be noticed that an addition of all of the x values in Table 1 was equal to 1.0, which was only 2% different from Oviously, the observed piezoelectric response in the multilayer structure of Al x As/GaAs was not from any particular layer but averaged over all the layers.

5 Vol. 25 No. 5 Sep.-Oct Figure 3. Avearage displacement over the frequencies ranged from 5 khz to 20 khz as a function of driving voltage for Al x As/GaAs with indium-gold electrode. Conclusion The Michelson-type interferometer was used in this work to measure the d 14 coefficient for GaAs and an accurate value of the coefficient was obtained to be (2.8±0.1) m/v. The technique was applied for investigating for the first time the piezoelectric effect in Al x As/GaAs and received a d 14 value of (3.9±0.1) m/v. By theoretical and experimental assessments, the multilayer structure sample showed the average piezoelectric response, which depended on the concentration of Al (or Ga) in the sample. Based on the direct piezoelectric effect, in which an electric field is induced by a strain, a similar response is anticipated to occur in this sample though an external electric field is zero. This is a consequence of the residual strains caused by some lattice constant mismatches in a structure with alternate layers of different materials (Bykhovski et al., 1995; Smith, 1997; Rees, 1997). Therefore, the piezoelectric effect must be properly accounted for in the design of strain-layered devices. Making use of this effect in deliberately strained structures to produce devices with desirable properties is possible (Disseix et al., 1999). Acknowledgements This work was financially supported by the 2001 Research Grant from Prince of Songkla University and the Graduate Fund from the Ministry of University Affairs. References Adachi, S GaAs, AlAs and Al x As: material parameters for uses in research and their applications, J. Appl Phys 58 (3): R1-R29. Adachi, S Physical Properties of III-V Semiconductor Compounds, John Wiley & Sons, New York. Arlt, G. and Quadflieg, P Pieozoelectricity in III-V compounds with a phenomenological analysis of the piezoelectric effect, Phys Stat Sol. 25: Blakemore, J.S Semiconducting and other major properties of GaAs J. Appl Phys 53(10): R123- R177. Bykhovski, A., Gelmont, B. and Shur, M Currentvoltage characteristics of strained piezoelectric structures, J Appl Phys 77(4): Charlson, E.J. and Mott, G The piezoelectric coefficient of gallium arsenide, Proc. IEEE 51: 1239.

6 Vol. 25 No. 5 Sep.-Oct Disseix, P., Ballet, P., Monier, C., Leymarie, J, Vasson, A. and Vasson, A.M Optical properties of (In,Ga)As/GaAs heterostructures grown on conventional (100) and (111)B GaAs substrate, Microelectron. J. 30: IEEE Standard on Piezoelectricity (ANS/IEEE Standard ) The Institute of Electrical and Electronics Engineerings, Inc., New York. Mason, W.P Physical Acoustics., Academic Press: New York. Muensit, S. and Kheanumkeaw, P Michelson interferometer for the piezoelectric coefficient measurements, Songklanakarin J. Sci. Technol. 24(1): Rees, G.J Strained layers piezoelectric semiconductor devices, Microelectron. J. 28: Shur, M Physics of Semiconductor Devices, Prentice-Hall International, Inc., USA. Smith, D.L Piezoelectric effects in strained layer heterostructures grown on novel index substrates, Microelectron. J. 28: Sze, S.M Physics of Semiconductor Device, John Wiley & Sons. Inc.: New York.

A phase mask fiber grating and sensing applications

A phase mask fiber grating and sensing applications ORIGINAL ARTICLE Suebtarkul Suchat 1 and Preecha P. Yupapin 2 Abstract Suchat, S. 1 and Yupapin, P.P. 2 Songklanakarin J. Sci. Technol., 2003, 25(5) : 615-622 This paper presents an investigation of a